In this tutorial, you will learn how to change the wallpaper for all your users at once. This has to be done from your administrator instance.
Set Default Background Wallpaper for Windows 10 Desktop Using Group Policy
Use the Windows key + R keyboard shortcut to open the Run command. Type gpedit.msc and press Enter.
Select User Configuration -> Administrative Templates -> Desktop -> Desktop. Double-click the Desktop Wallpaper policy on the right side.
Select the Enabled option, and then type the path of the image you want to set as a default background. Click Apply and OK.
Right click on the Start Menu Button and select command prompt or command prompt (Admin) to open the CMD window. Type: gpupdate /force and hit ENTER.